Conventional abrasive blasting equipment has long been used in various industries to remove coatings such as paint or to clean grease, dirt and the like from the surfaces of a variety of composite materials or work pieces. Such conventional blasting equipment generally includes a media hopper for loading particles of an abrasive blasting medium such as sand, a pressure vessel or blast pot to hold the particles of the abrasive blasting medium, and a feed valve for metering the amount of abrasive blasting medium to be emptied into a high velocity compressed air stream which flows through a conveying hose connected to the source of compressed air. The air stream within the conveying hose passes through the metering device to a blast hose which transports the particles of blasting medium toward the nozzle of the hose which is directed toward the work piece to be cleaned or stripped. The particles of blasting medium are accelerated and dispelled from the nozzle at such a high velocity that the coating or dirt on the work piece is removed.
However, some substrates such as aluminum, magnesium, and certain types of plastic composites may be damaged or destroyed by standard abrasive media such as sand. For instance, it is well known that conventional sandblasting equipment cannot be used to clean dirty and greasy gear drives, mechanical valves, piping, glass gauges and reservoirs thin gauge metal covers and shields or painted galvanized surfaces and the like which are typically employed in the chemical or petrochemical industry. Accordingly, less aggressive abrasives such as sodium chloride or sodium bicarbonate must be used. However, conventional abrasive blasting equipment does not have a fast, sufficient or effective means for conveying media such as sodium bicarbonate from the pressure vessel through the blast hose to the nozzle.
In particular, many metering devices or feed valves employed in conventional blasting apparatuses are aerated such that a "fluidized bed" on the inside of the pressure vessel is created which enables the media to flow like a liquid through an orifice into the compressed air stream. However, conventional feed valves are equipped with and only provide for a three-quarters of an inch (3/4") inside diameter tube for the compressed air stream to flow through. Thus, while this diameter is sufficient for some media such as sand, it is not sufficient for less aggressive blasting media such as sodium bicarbonate. Specifically, when sodium bicarbonate media is used as the blasting media, a three-quarters of an inch inside diameter tube at the feed valve requires the operator to use a hose nozzle having only one-quarter inch (1/4") inside diameter because the blasting media must be delivered to the air stream from the pressure vessel at a rate complementary to the rate at which the air is flowing. Thus, a small diameter tube at the feed valve does not permit the nozzle to spray the abrasive blasting media at as fast a rate in cubic feet per minute (CFM) as a larger diameter tube or shaft. Moreover, such a small nozzle can clean or strip only a small area on a substrate at any one time. In most instances using sodium bicarbonate, it is much more desirable to use up to a one-half inch (1/2") nozzle. In order to do this, however, a one and one-quarter inch (11/4 ") inside diameter air hose and feed tube through the feed valve assembly are required to accommodate the increased CFM requirement.
It is important to note at this point that due to the structure of the body casing of the feed valve assembly and the manner in which the feed valve is fabricated, a larger diameter feed tube cannot be employed with a conventional feed valve assembly. Significantly, a conventional aerated-type feed valve includes a hollow body casing having a hollow feed tube glued within the hollow shaft portion of the body casing through which compressed air may flow. A fixed orifice or feed stem may be threadably received by the body casing vertical to the hollow shaft portion such that the orifice communicates with the hollow feed tube. That is, the orifice has a centrally disposed bore therethrough which is in alignment with a bore disposed through the feed tube so that the blasting media may be received through the bore in the orifice and the feed tube and enter into the compressed air stream. At one end of the shaft portion of the body casing, a coupling for attaching the blast hose conveying the media and air stream to the nozzle may be attached by threadably receiving the shaft portion of the body casing, not the feed tube itself. Because of this, the body casing cannot, and until now, has not been made larger.
Other types of blasting apparatuses are capable of using sodium bicarbonate with larger nozzles. However, these apparatuses do not use aerated-type feed valves and are not very useful with more aggressive blasting media such as sand. For example, Kirschner et al. U.S. Pat. Nos. 5,081,799 and 5,083,402 both relate to industrial pressure blasting apparatuses which have been modified to allow a controlled pressure on the blast pot or pressure vessel that is greater than the pressure on the air stream line. Accordingly, these patents use at least two regulators, and the pressure in the blast pot or pressure vessel essentially forces the sodium bicarbonate or other relatively nonabrasive blast media into the compressed air stream. The media is not made to flow like a liquid as is essentially the case with aerated-type feed valves, but rather is vibrated vigorously to loosen any "clumps" and to force the media through an orifice disk.
In addition, when standard media fill hoppers are attached to conventional pressure vessels, yet another valve assembly is required to seal the pressure vessel during use. Accordingly, most abrasive blasting equipment includes a dump valve assembly to seal the pressure vessel during operation. However, in order to close the valve so that the blasting apparatus may function properly, at least about 30 to 40 PSI of air pressure is required. This is caused by the fact that the air used to operate this valve is required to lift a weighted down dump valve ball in order to seal the pressure vessel. This does not allow conventional blasting equipment to operate at lower blast pressures of between about 5 and 10 PSI.
Thus, the need exists for a blasting apparatus which includes an aerated-type feed valve assembly which has a large enough compressed air stream feed tube or shaft to permit the operator to use up to a one-half inch (1/2") nozzle with all types of abrasive blasting media including sodium bicarbonate and a dump valve assembly which requires only minimal air pressure to close the dump valve and seal the pressure vessel, thereby allowing the apparatus to function properly at low pressure.